The present invention relates to rotary fluid pressure devices of the type in which a gerotor gear set typically serves as the fluid displacement mechanism, and more particularly, to such devices which are provided with multiple-speed (multiple-displacement) capability. Furthermore, the present invention relates to an improved method for controlling the shifting (between different speed ratios) of such a multiple-speed device.
Although the teachings of the present invention can be applied advantageously to devices having fluid displacement mechanisms other than gerotor gear sets (such as radial piston and cam lobe type devices), the present invention is especially adapted for use with devices utilizing gerotor gear sets, and will be described in connection therewith. Furthermore, the present invention is especially adapted for devices which serve as motors during most of their operating cycle, and will be described in connection therewith.
Motors utilizing gerotor gear sets can be used in a variety of applications, one of the more common applications being vehicle propulsion, wherein the vehicle includes an engine driven pump which provides pressurized fluid to a vehicle hydraulic propel circuit, including a pair of gerotor motors, with each motor (typically but not necessarily) being associated with one of the drive wheels. Those skilled in the art will understand that many gerotor motors utilize a roller gerotor gear set, especially on larger, higher torque motors of the type typically used in propel applications, and subsequent references hereinafter to a xe2x80x9cgerotorxe2x80x9d will be understood to mean and include both a conventional gerotor as well as a roller gerotor. For purposes of this invention, xe2x80x9cgerotorxe2x80x9d can include either an IGR (internally-generated rotor) or and EGR (externally-generated rotor), both of which are now generally well known to those skilled in the art.
Multiple-speed gerotor motors are known from U.S. Pat. Nos. 4,480,971; 6,068,460; and 6,099,280, all of which are assigned to the assignee of the present invention and incorporated herein by reference. The device of the ""971 patent has been in widespread commercial use and has performed in a generally satisfactory manner, and more recently, the devices of the ""460 and ""280 patents have also come into commercial usage. As is now well know to those skilled in the art, a gerotor motor may be operated as a multiple-speed ratio (multiple displacement) device by providing valving which can effectively xe2x80x9crecirculatexe2x80x9d fluid between expanding and contracting fluid volume chambers of the gerotor gear set. While the inlet port communicates with all of the expanding volume chambers, and all of the contracting volume chambers communicate with the outlet port, the motor operates in the normal, low-speed, high-torque (LSHT) mode or condition. When some of the fluid from certain of the contracting volume chambers (the xe2x80x9crecirculatingxe2x80x9d chambers) is recirculated back to the expanding volume chambers, the result will be operation in a high-speed, low-torque (HSLT) mode or condition. The HSLT mode yields the same result as if the displacement of the gerotor gear set were decreased, but with the same fluid flow rate through the gerotor.
The multiple-speed gerotor motors, made in accordance with the above-incorporated patents, and sold commercially by the assignee of the present invention, operate very satisfactorily in both the LSHT and the HSLT modes. It has been observed, however, that when the motor is shifted from one mode to the other (and especially, from the HSLT mode to the LSHT mode), there is a tendency for cavitation to occur in the gerotor gear set, just as the shift is occurring from one mode to the other. During the shift from HSLT to LSHT, the effective xe2x80x9cdisplacementxe2x80x9d of the motor increases, while the speed of the vehicle and the pump flow remain, at least in the short term, generally constant. Thus, the gerotor gear set is suddenly being xe2x80x9cdisplacedxe2x80x9d at a speed corresponding to an instantaneous fluid flow rate which is greater than what the pump can immediately provide.
The recirculating fluid volume chambers have the greatest tendency to cavitate because of greater restriction in the recirculation flow path than in the flow paths to and from those volume chambers which operate normally (don""t recirculate). As is well know to those skilled in the art, cavitation occurring within a fluid displacement element, such as a gerotor gear set, causes a substantial amount of undesirable noise, and can also eventually result in damage to the displacement mechanism. Typically, the cavitation will continue until the vehicle slows down to a speed at which the pump flow xe2x80x9ccatches up withxe2x80x9d the speed (displacement) of the gerotor gear set in the motor.
Another problem which has been observed in connection with the process of shifting (again, especially from the HSLT mode to the LSHT mode), is that, if the shift is accomplished too quickly on a vehicle, for example, one moving a load, there will be a tendency for the load to keep moving under its own momentum, even as the vehicle slows down. Thus, there is the potential danger of losing at least part of the load. Finally, the sudden slowing of the vehicle has, on a number of occasions, been observed to cause skidding of the vehicle which, if repeated many times, can result in excessive tire wear.
Accordingly, it is an object of the present invention to provide an improved fluid pressure operated device having multiple-speed ratio capability, in which shifting from one mode to another does not result in any substantial amount of cavitation and noise.
It is a more specific object of the present invention to provide an improved method for controlling the shifting of a multiple-speed ratio fluid pressure operated device, wherein the shifting occurs without any substantial occurrence of the problems associated with the prior art as described above.
It is another object of the present invention to provide an improved method for controlling the shifting of a multiple-speed ratio fluid pressure operated device, wherein each different type of shifting operation can be achieved in a manner most appropriate for that particular shifting operation.
The above and other objects of the invention are accomplished by the provision of an improved method of controlling the shifting of a multiple-speed ratio fluid pressure operated device between a first speed ratio and a second speed ratio, the device including a fluid pressure displacement mechanism defining a plurality of expanding and contracting fluid volume chambers. A motor valve means is operable to provide fluid communication to and from the fluid volume chambers in the first speed ratio. A shift valve means is operable, in a first condition, to achieve the first speed ratio, and in a second condition, to achieve the second speed ratio by interconnecting a plurality of the volume chambers as recirculating volume chambers. The method comprises the step of shifting the shift valve means between the first and second conditions in response to changes in a pilot pressure signal between a first pressure and a second pressure.
The improved method is characterized by providing a pressure control valve in fluid communication with a source of pressurized fluid, the pressure control valve being operable to communicate the pilot pressure signal to the shift valve means in response to changes in an electrical command signal between a first signal and a second signal. When a shift to the second condition is commanded, the method includes changing the electrical command signal from the first signal to the second signal over a first time period T1. When a shift to the first condition is commanded, the method includes changing the electrical command signal from the second signal to the first signal over a second time period T2, wherein T2 is greater than T1.